The present invention relates to a wire sawing device comprising at least two wire guide cylinders supporting a wire held in position by grooves provided on the surface of said wire guide cylinders which define the spacing between the wires of a layer of wires, the wire being adapted to move reciprocatingly or continuously while bearing against a piece to be sawed fixed to a supporting table, the winding of the wire about the wire guide cylinders generating at least two layers of wires that cross along at least one line at an angle greater than zero, the wire being wound about each of the wire guide cylinders over an angle of at least 120xc2x0 of the circumference of the cylinder.
During cutting of thin layers from the piece to be sawed, the stretched wire is both guided and driven by the wire guide cylinders. These wire guide cylinders generally coated with a synthetic layer are engraved with grooves whose shape and dimensions must be quite precise. The wire is spirally wound about two or more wire guide cylinders and forms between two wire guide cylinders at least one layer of parallel wires whose spacing between adjacent wires determines the thickness of the slices. Because of the spiral winding, all the wires of the layer of wires move parallel, generating a force perpendicular to the advance of the supporting table and imposing shear and torsion forces on the mechanism for holding the piece to be sawed. These forces are a function of the speed of sawing and of the dimensions of the piece to be sawed and can result in carrying off the piece to be sawed from its support. The tensions produced at the base of the piece to be sawed lead to deformations which after sawing show us as warping of the obtained slices. Moreover, the plane of the layer of wires is in the known devices such as, for example the one described in the document No. 91/12915, generally perpendicular to the direction of sawing, which can lead to waves on the surfaces of the slices in the case of general movement of the layer of wires resulting from thermal variations for example. These waves, although of several micrometers, are great enough to render the slices unusable for certain applications such as silica for the semiconductor industry.
The document JP-61-288995 discloses a device comprising the characteristics set forth in the preamble of the independent claim. In this known device, the winding of the wire about the wire guide cylinders generates two layers of wires which cross at an angle than zero. To obtain parallelism between the wires of one layer relative to those of the other layer in the direction of movement of the piece to be sawed, the known device comprises a series of guide rollers interspaced between the wire guide cylinders.
The presence of these rollers renders the construction complicated and of a largely greater size. Considerable forces are moreover generated at the level of these guide rollers which are subjected to rapid wear.
The present invention has for its object to overcome these drawbacks and to provide a sawing device with at least two layers of crossing wires and which permits obtaining sawed slices with parallel and flat surfaces without the use of guide rollers.
The device according to the present invention is characterized to this end by the fact that it comprises means to carry out relative displacement between the piece to be sawed and the layers of wires in a direction of relative movement and adjustment means to adjust the angle of inclination comprised between said direction of said relative movement and a perpendicular to a working plane in which are contained the axes of two adjacent wire guide cylinders, this angle of inclination being determined and adjusted so as to obtain slices with parallel surfaces.
With these characteristics, there is obtained a sawing device of a less complicated construction and generally reduced size, whilst permitting the sawing of slices with parallel and flat surfaces. By adjusting precisely said angle of inclination, perfect parallelism of the surfaces of the sawed slices can be obtained without resorting to guide rollers.
There are thus used at least two layers of wires making a sufficient angle with the direction of cutting. The crossing of the layers relieves the problem of waves because these latter will no longer be parallel but will be of different angle on opposite sides of the obtained slices, which gives the possibility of a subsequent correction by lapping or by truing. The use of two wire guides according to a preferred embodiment in the wire sawing device permits providing in a simple manner, by winding of the crossing wire in figure 8 configuration between the two wire guides, the two directions of sawing having a sufficient angle to create the required effect to diminish the problem of possible waves. It is particularly important to note that this configuration, the shear stresses induced in the support by the two layers having different angular directions from the direction of sawing, hence the advance of the support table, are considerably reduced. Moreover, the angular position of the layers of wires relative to the movement of the support table, and hence of the piece to be sawed, increases the cutting pressure on the surface of the piece to be sawed and thus permits a higher speed of cutting, increasing at the same time the productivity of the sawing device per wire. The necessary angle between the two layers should be at least 20xc2x0 if it is desired to obtain substantial crossing effect. However, beyond 90xc2x0, the sawing time begins to decrease the economic interest in the use of crossing layers.
Because the wire is wound about each of the wire guide cylinders about at least 120xc2x0 of the circumference of the cylinder and passes in a straight line from one guide cylinder to another, and hence without passing by a guide member such as a guide cylinder or deflector, there is obtained a high crossing angle of the layers and a very good alignment and optimum parallelism of the wires of a layer. The invention therefore permits providing a high performance wire sawing device having high productivity, improved average precision by the decrease of geometric irregularities and the decrease of internal tensions in the piece to be sawed (which is often a material highly sensitive to external forces) in the course of sawing.
Preferably, this angle of inclination is determined and adjusted as a function of the distance separating two adjacent grooves on the wire guide cylinders, the diameter of these latter, and the distance separating these latter.
During changing one of these three parameters, the occasion will arise to adjust the angle of inclination, which will again permit obtaining slices with exactly parallel surfaces.
Other advantages will appear from the characteristics set forth in the dependent claims and from the expository description hereinafter of the invention in greater detail, referring to the drawings, which show schematically and by way of example one embodiment and modifications.